You are aiming to spend $100 or less on meat for a barbeque. Your options are chicken or steak. Chicken costs $1.29/lb and steak costs $3.49/lb. How much of each meat can be purchased?
If you wish to spend about 50% of your money on chicken and about 50% on steak, then you have:
$50 / $1.29 =~39 Lbs of chicken
$50 / $3.49 =~14 Lbs of steak.
[39 x $1.29] + [14 x $3.49] = $99.17 - total amount of money spent.
$100 - $99.17 = $0.83 - money left over.
Note: if you want to spend all of $100 on either chicken or steak, you would simply divide $100 by $1.29 =~77 Lbs of chicken. Or, $100 divided by $3.49 =~28 Lbs of steak. In either case, you will have:
$0.67 cents and $2.28 respectively left over. With $2.28 you could buy one extra pound of chicken for $1.29, which will leave you with $0.99 cents left over.
Say you buy no steak at all then all the money goes on chicken 100/1.29 = 77.5 lbs of chicken
Say you buy no chicken and all the money goes on steak 100/3.49 = 28.6 pounds of steak.
I can graph this. You can purchase any amount of the 2 types of meat that lies inside the triangle